Field of the Invention:
The present invention relates to compounds comprising a redox group, to their use as additive in an electrolyte composition, to an electrolyte composition comprising said additive and to electrochemical systems comprising said electrolyte composition, in particular lithium or sodium batteries and electric double-layer supercapacitors.
Description of Related Art:
In a known way, batteries are composed of a positive electrode (cathode), generally a transition metal oxide (cobalt or manganese dioxide), and of a negative electrode made of graphite (anode), between which is placed a separator impregnated with an electrolyte consisting of a lithium or sodium salt in solution in a solvent chosen in order to optimize the transportation and the dissociation of the ions (in general a mixture of carbonates). A current collector is connected to the cathode in order to ensure the electrical connection.
Electrochemical systems having electric double-layer and/or pseudocapacitive storage, such as supercapacitors, are energy storage devices, the basic principle of which is based on the capacitive properties of the interface between a solid electron conductor and a liquid ion conductor. A supercapacitor is generally composed of two metal current collectors, generally made of aluminum, of two porous carbon-based electrodes impregnated with electrolyte, and of a porous separating membrane. Energy storage is carried out by distributing the ions of electrolyte in the vicinity of the surface of each electrode under the electrostatic influence of the applied voltage. A space charge region, known as electric double layer, with a thickness limited to a few nanometers and in which the relatively intense electric field (of the order of 10 kV·μm−1) prevails, is thus created at the interfaces.
Electrochemical systems having faradaic electrodes, such as lithium batteries, exhibit good performances in terms of energy but are not very effective in terms of power. The reverse is the case for electrochemical systems having double-layer and/or pseudocapacitive storage, such as supercapacitors, for example. Furthermore, electrochemical systems having faradaic electrodes, such as lithium batteries or hybrid supercapacitors, for example, all operate by virtue of an electrode in which the electrode active material has to be (co)mixed beforehand with one or more polymers or deposited with/on the conducting agent or agents of the electrodes before assembling the electrochemical system, so as to obtain effective electrical contacts, a condition sine qua non for good performances of these systems. It is the stage of shaping the electrode.
Various methods have already been envisaged for improving the performances, in particular power, of these electrochemical systems.
Some authors, such as Qin Y. et al. (Journal of Power Sources, 2010, 195, 6888-6892), have tested the addition of 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane (TOS) to a base electrolyte in order to increase the lifetime and the thermal stability of lithium batteries comprising a negative electrode based on mesocarbon microbeads (MCMBs), on carbon fibers and on polyvinylidene fluoride (PVDF) dispersed in N-methyl-2-pyrrolidone on a copper sheet, a positive electrode based on Li[Ni1/3Co1/3Mn1/3]O2 (NCB), on carbon black and on Pilaf dispersed in N-methyl-2-pyrrolidone on an aluminum sheet, a separator made of microporous polypropylene and LiPF6 as electrolyte. The authors indicate that the addition of TOS in a proportion of 1% by weight to the electrolyte improves the retention capacity of the battery (in operation at a temperature of 55° C.) and its thermal stability but that an amount of this same compound of greater than 0.5% brings about, on the other hand, a significant fall in the impedance.
Yu H. et al. (Electrochemical and Solid-State Letters, 2004, 7(11), A442-A446) have tested the use of vinyl ethylene carbonate as additive in an electrolyte based on propylene carbonate and on lithium bis(perfluoroethylsulfonyl)imide (LiBETI) in lithium batteries comprising either an anode prepared from a graphite powder or an anode prepared from MCMB. The presence of this additive in the electrolyte makes it possible to create a protective film at the surface of the electrode, so as to prevent the lithium ions from becoming inserted in the graphite and from damaging it, which has the consequence of improving the performance of the battery. However, the authors indicate that this effect is only observed with the battery comprising the electrode prepared from graphite powder but not with that in which the electrode was prepared from MCMB. This method is therefore not applicable to every type of carbon-based electrode and even less to any type of electrode.
Other methods employ redox shuttles, that is to say batteries in which the electrolyte comprises, as additive, a redox compound or a reactive polymer which, during the operation of the battery, plies between the two electrodes (Z. Chen et al., Electrochimica Acta, 2009, 54, 5605-5613), with the aim of preventing overcharges. Such redox compounds can, for example, be aromatic compounds, such as 1,4-di(tert-butyl)-2,5-dimethoxybenzene, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) or 2-(pentafluorophenyl)tetrafluoro-1,3,2-benzodioxaborole (PFPTFBB), or nonaromatic compounds, such as certain borated lithium salts, such as Li2B12F12−xHx with x=1-12. However, the authors indicate that it remains difficult to find compounds which do not end up by decomposing and which remain effective over time. Furthermore, these compounds have to diffuse into the electrolyte in order to ensure redox reversibility. They are thus not localized at an electrode, which renders them incapable of delivering and/or of accumulating high-power energy.
Finally, other methods provide for the functionalization of the carbon electrodes by organic groups of carboxylic or amino type, such as described, for example, by Lee et al. (Nature Nanotechnology, 2010, 5, 531-537), by sulfophenyl groups, as described by D. Pech et al. (Electrochemical and Solid-State Letters, 2008, 11, A202-A205), or by aromatic groups, for example of pyrene type, such as described in particular by C. Ehli et al. (JACS, 2006, 128, 11222-11231). However, these methods make it necessary to carry out the molecular functionalization of the electrode active material (graphite or carbon) before the assembling of the nonconventional electrodes, which thus necessitates an additional stage.